The present invention relates to a multicellular structure, such as a honeycomb, particularly for trapping and combusting diesel exhaust particulates.
Wall-flow filters are used in the purification of diesel exhaust. Typically such diesel particulate filters are made of cordierite or silicon carbide and include a honeycomb body having thin interconnecting porous walls which form parallel cell channels of equal hydraulic diameter, longitudinally extending between the end faces of the structure. Alternating cells on one end face of the honeycomb are plugged with a ceramic filler material to form a “checkerboard” pattern. The pattern is reversed on the opposite side, so that the ends of each cell are blocked at only one end of the structure. When diesel exhaust enters the filter through one end face (i.e., inlet end), it is forced to pass through the porous walls in order to exit through the opposite end face (i.e., outlet end).
For diesel particulate filtration, honeycomb structures having cellular densities between about 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2), are considered useful to provide sufficient thin wall surface area in a compact structure. Wall thickness can vary upwards from the minimum dimension providing structural integrity of about 0.002 in. (about 0.05 mm.), but are generally less than about 0.060 in. (1.5 mm.) to minimize filter volume. A range of between about 0.010 and 0.030 inches (about 0.25 and 0.76 mm.) e.g., 0.019 inches, is most often selected for these materials at the preferred cellular densities.
Interconnected open porosity of the thin walls may vary, but is most generally greater than about 25% of thin wall volume and usually greater than about 35% to allow fluid flow through the thin wall. Diesel filter integrity becomes questionable above about 70% open pore volume; volumes of about 50% are therefore typical. For diesel particulate filtration it is believed that the open porosity may be provided by pores in the channel walls having mean diameters in the range of about 1 to 60 microns, with a preferred range between about 10 and 50 microns.
Filtration efficiencies up to and in excess of 90% of the diesel exhaust particulates (by weight) can be achieved with the described cordierite materials. The filtration of a lesser but still significant portion (i.e. less than 50%) of the particulates may be desirable for other filtering applications including exhaust filtering of smaller diesel engines. Efficiencies, of course, will vary with the range and distribution of the size of the particulates carried within the exhaust stream. Volumetric porosity and mean pore size are typically specified as determined by conventional mercury-intrusion porosimetry.
U.S. Pat. No. 4,420,316 to Frost et al. discusses cordierite wall-flow diesel particulate filter designs. U.S. Pat. No. 5,914,187 discusses silicon carbide wall-flow diesel particulate filters.
There are problems associated with conventional filters of the type described therein. Specifically, as the exhaust passes through the filter, particulate matter (i.e., carbon soot) accumulates on the wall of the cell channels or in the pores of the wall and forms a soot layer. This soot layer decreases the hydraulic diameter of the cell channels contributing to a pressure drop across the length of the filter and a gradual rise in the back pressure of the filter against the engine.
Eventually, the pressure drop becomes unacceptable and regeneration of the filter becomes necessary. In conventional systems, the regeneration process involves heating the filter to initiate combustion of the carbon soot layer. Normally, during regeneration the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1000° C. Under certain circumstances, a so-called “uncontrolled regeneration” can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas (such as engine idling conditions). During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter. The highest temperatures during regeneration tend to occur near the exit end of the filter due to the cumulative effects of the wave of soot combustion that progresses from the entrance face to the exit face of the filter as the exhaust flow carries the combustion heat down the filter.
In addition to capturing the carbon soot, the filter also traps “ash” particles that are carried by the exhaust gas. These particles which include metal oxide impurities, additives from the lubrication oils, sulfates and the like, are not combustible and, cannot be removed during regeneration. Furthermore, if temperatures during uncontrolled conditions are sufficiently high, the ash particles may eventually sinter to the filter or even react with the filter resulting in partial melting.
It has been recognized that, since in operation the soot built-up reduces the effective flow area of the cell channels, this collection of particulates forming a cake on the cell wall surfaces and in the pores, ultimately also affects the pressure drop. One approach for addressing this problem has been proposed in U.S. Pat. No. 4,276,017 issued to Outland on Jun. 30, 1981. Outland discloses a filter design having cross-sectional areas of the inlet channels larger than the cross-sectional areas of the adjacent outlet or exhaust channels. To achieve the non-equal area inlet and outlet passages, Outland teaches non-equilateral hexagonal cross-section inlet cell channels adjacent triangular cross-section outlet cell channels, as illustrated in FIGS. 5h-j, and patterns with curving or bulging inlet and/or outlet channels, as illustrated in FIGS. 5k-p. In the design of the filter structure, Outland also requires that all internal walls extend between inlet and outlet channels except at their points of intersection, such as at the corners. However, such designs are not practical and more importantly difficult to manufacture in an efficient and cost-effective manner. Current technology die limits the fabrication of such complex honeycomb structures.
It would be desirable to obtain a diesel exhaust particulate filter that exhibits increased carbonaceous soot and ash particulate storage capacity while at the same time maintaining good gas flow rates. It would also be desirable to manufacture such a filtering device efficiently and cost-effectively with current die technologies.